1. Field of the Invention
The invention relates to a system and a method for gamma correction, and more particularly to which apply a single ramp voltage to undertake a dynamic gamma correction of non-uniform frequency clocks and method therefor.
2. Related Art
Usually, a display doesn't generate luminance linearly. Therefore gamma curve correction is required in order to obtain the required luminance. In old days of CRT monitors, brightness (B) relates to the voltage generated by the electronic gun by being proportional to the gamma (γ) order of the voltage (Vs), which forms a famous γ-curve. At that time, signal transmitted by the TV station must correspond to this γ-curve, so that the brightness/darkness ratio and the color performance of the image can be correct. For the same reason, today's updated LCD monitor also needs to comply with the γ-curve.
In IT era, everything is standardized, including the γ-curve for a value of 2.2 or 2.4. However, people may feel differently about an image showed on TV. Different people may prefer different stronger, lighter, brighter or darker color performance with the same image, which means they may have their own γ value preference. And adjusting the γ-curve can produce different color and brightness performance.
Usually, details in a dark image hardly can be identified. Although brightness can be increased in whole to make the dark area more clear, the image may lose its reality. For example, the color of blue sky may fade. Hence, if the γ-curve is capable of changing in part, the contrast ratio can be increased by part of the brightness is corrected.
One method for a conventional LCD to adjust the γ-curve is using a resistor co-working with a buffer to divide the reference voltage to achieve the gamma correction. A plasma display panel is using a high voltage data driver and controlling the grayscale by applying a uniform counter clock to produce the required grayscale. The produced corresponding γ-curve is similar to an exponent curve therefore can not represent the real grayscale. Other methods for gamma correction include applying multiple ramp voltage waveforms or PWM.
U.S. Pat. No. 6,137,462 has disclosed a known gamma correction method, where a LCD driving circuit is disclosed. The main technical feature is to design multiple ramp voltages (ramp waveforms) based on the T-V curve, and co-works it with a counter by a ramp voltage (ramp waveform) selector, so the time for the input image data can be adjusted for selecting a voltage, which corresponds to the input data to achieve the brightness-voltage linear correction.
US published application US20040090402 has disclosed another known gamma correction method, where a method and an apparatus for gamma correction for displays are disclosed. The main technical feature is to undertake the gamma correction by co-working a produced non-linear ramp voltage (ramp waveform) with Supertex's HV623 driver IC.
US published application US20040135778 has disclosed another known gamma correction method, where a display is disclosed, and the gamma correction is proceeded by using a reference data generating circuit to determine the counting frequency by comparing the value of the counter and a predetermined value. The method uses a single ramp voltage (ramp waveform) and a predetermined multi frequency check table to determine whether to perform gray control by way of look up the table. This technique utilizes fixed, predetermined, and limited multi frequency to perform the gamma correction.
The forgoing mentioned brightness-voltage curves are all non-linear. Therefore a gamma correction is necessary to obtain rich and correct color. However, the circuit of conventional multi ramp voltage (ramp waveform) for gamma correction is complex and high cost. Besides, since a high bandwidth driver IC is necessary for a conventional PWM gamma correction, the cost and EMI are both high, either.
Therefore, utilizing single ramp voltage (ramp waveform) and non-uniform frequency clocks counter clock to control grayscale is probably a good way to cost down in design. The frequency of the counter clock can be calculated based on the curve slope of brightness-voltage in order to obtain a linear gamma curve. Since the frequency of the counter clock is obtained by calculation, it is non fixable and unlimited adjustable so the linearity of the gamma curve can be infinitely increased.
According to the forgoing problems, the invention provides a low cost and high performance solution.